 They convert a phase-keyed signal into an amplitude-keyed signal.In this application, an incoming differential phase-shift keying (DPSK) optical signal is first split into two equal-intensity beams in two arms of a Mach Zehnder or Michelson interferometer, in which one beam is delayed by an optical path difference corresponding to "1-bit" time delay.After recombination, the two beams interfere with each other constructively or destructively.